Fix qBittorrent service not reloading config after settings change

Root cause: Singleton caching issue
The qBittorrent service uses a singleton pattern with a configLoaded flag.
Once initialized, it NEVER re-reads the database config, even when the
user changes settings via the admin settings page.

Flow showing the bug:
1. Wizard saves download_dir to database ✓
2. First torrent: service reads config, creates singleton, sets configLoaded=true ✓
3. User changes download_dir in settings page ✓ (database updated)
4. Next torrent: getQBittorrentService() returns CACHED singleton ✗
5. Cached singleton has OLD download_dir value in this.defaultSavePath ✗
6. Category check shows "already has correct save path: /old/path" ✗
7. Download goes to wrong location ✗

The singleton check (line 745):
  if (!qbittorrentService || !configLoaded) {
    // Only runs if service doesn't exist or config failed
  }
Once both exist, this block is SKIPPED forever!

Fix:
1. Added invalidateQBittorrentService() function
   - Resets qbittorrentService = null
   - Resets configLoaded = false
   - Forces reload from database on next use

2. Call invalidation from settings APIs:
   - After updating paths (download_dir, media_dir)
   - After updating download client (URL, credentials)

3. Next torrent addition:
   - getQBittorrentService() sees null singleton
   - Re-reads config from database
   - Creates new service with current download_dir
   - Category updated with correct path

Benefits:
- Settings changes take effect immediately
- No app restart needed
- Category save path always matches current config
- Download client credentials always current

Updated documentation to explain singleton invalidation pattern.

src/app/api/admin/settings/paths/route.ts

@@ -55,6 +55,10 @@ export async function PUT(request: NextRequest) {
 
         console.log('[Admin] Paths settings updated');
 
+        // Invalidate qBittorrent service singleton to force reload of download_dir
+        const { invalidateQBittorrentService } = await import('@/lib/integrations/qbittorrent.service');
+        invalidateQBittorrentService();
+
         return NextResponse.json({
           success: true,
           message: 'Paths settings updated successfully',